Alterations in oxygen tension are known to affect gene expression in all cell types. In bacterial systems, oxygen affects expression of a variety of genes involved in aerobic versus anaerobic energy generation such as nitrate reductase and cytochrome oxidase. The presence of oxygen reactive species, also affects expression of genes involved in defense against oxidative damage. A variety of additional metabolic processes such as carbon fixation, nitrogen fixation, and anoxygenic photosynthesis are also regulated in response to the presence or absence of oxygen. In yeast and algal cells, oxygen is known to affect transcription of oxidative defense genes as well as enzymes involved glycolysis. In mammalian cells, a growing number of genes are known to be oxygen regulated such as those coding for growth factors such as erythropoietin and vascular endothelithial growth factors that are key regulators for the synthesis of new capillary sprouts from preexisting vessels. Besides physiological roles, these growth factors are also involved in disease processes such as the stimulation of capillary formation during tumor growth. This proposal is centered on elucidating molecular details of how oxygen affects gene expression in bacteria. As a model system, we study oxygen regulated gene expression in Rhodobacter capsulatus which is closely related to the mitochondrial lineage. This species is capable of growth in a variety of energy generating modes including aerobic respiration, anaerobic fermentation and photosynthesis. The expression of genes involved in each of these processes is known to be affected by alterations in oxygen tension and well as by variations in light intensity. As such, this organism offers itself as an important model system to the study oxygen and light regulation of gene expression in a number of biological systems.